(1) Summary of the Invention
The present invention relates to the preparation of alkali metal chalcogenides of bismuth alone or with antimony. In particular, the present invention relates to chalcogenides with a unique combination of properties at temperatures -100 to 150.degree. C.
(2) Description of Related Art
Since the solid solutions of Bi.sub.2-x Sb.sub.x Te.sub.3-y Se.sub.y (Jeon, H.-H., Ha, H.-P., Hyun, D.-B., Shim, J.-D., J. Phys. Chem. Solids 4 579-585 (1991); Testardi, L. R., Bierly, J. N. Jr., Donahoe, F. J., J. Phys. Chem. Solids 23 1209 (1962; Champness, C. H., Chiang, P. T., Parekh, P. Can. J. Phys. 43 653-659 (1965); and 45, 3611-3626 (1967)) were established as the leading materials available for near-room-temperature thermoelectric applications, there have been continuing efforts to find better thermoelectric materials. The challenge lies in achieving simultaneously high electrical conductivity, high thermoelectric power, and low thermal conductivity. These properties define the thermoelectric figure of merit ZT=(S.sup.2 .sigma./.kappa.)T, wherein S is the thermopower, .sigma. the electrical conductivity, .kappa. the thermal conductivity, and T the temperature. All three of these properties are determined by the details of the electronic structure and scattering of charge carriers (electrons or holes) and thus are not independently controllable parameters. .kappa. also has a contribution from lattice vibrations, .kappa..sub.l, the phonon thermal conductivity. Thus .kappa.=.kappa..sub.e +.kappa..sub.l, where .kappa..sub.e is the carrier thermal conductivity. To date, most investigations were mainly focused on tuning (Yim, W. M., J. Electrochem. Soc. 115 556-560 (1968; Yim, W. M., et al., J. Mater. Sci. 1 52-65 (1966; and Borkowski, K., et al., J. Mater. Res. Bull. 22 381-387 (1987)) the composition of Bi.sub.2 Q.sub.3 (Q=S, Se, Te) solid solutions, doping (Chizhevskaya, S. N., et al., Inorg. Mater. 31 1083-1095 (1995); Horak, J., et al., J. Phys. Chem. Solids 47 8805-809 (1986; Lostak, P., et al., Phys. Status Solidi 76, k71-k75 (1983); and Zalar, S. M., Adv. Energy Conv. 2 105-112 (1962)) Bi.sub.2 Q.sub.3 with other heavy metals, and optimizing device design.
From a solid-state chemistry perspective, an intriguing feature of Bi/Sb chemistry is the stereochemical localization of their ns.sup.2 lone-pair electrons, and the influence this exerts on the structure type and the electronic structure, and consequently the electronic properties of the resulting compounds. In this sense, alkali or alkaline earth metals introduced into the Bi.sub.2 Q.sub.3 lattices rearrange the octahedrally coordinated Bi/Sb elements often causing the group 15 element (Sb or Bi) to exhibit varying degrees of ns.sup.2 lone-pair stereochemical activity. In addition to the multitude of naturally occurring sulfosalt minerals, several synthetic ternary alkali or alkaline earth metal group 15 chalcogenides are known such as ABQ.sub.2 (A=alkali metal; B=group 15 metal; Q=chalcogen), (Boon, J. W., Recl. Trav. Chim. Pays-Bas 63 32 (1944); Glemser, O., et al., Anorg. Allg. Chem. 279 321-323 (1955); Gattow, G., et al., Anorg. Allg. Chem. 279 324-327 (1955); and Voroshilov, Y. V., et al., Inorg. Mater. 8, 777-778 (1972)) CsBi.sub.3 S.sub.5 (Kanischeva, A.
S., et al., Kokl. Adad. Nauk. SSSR (Kryst.) 252, 96-99 (1980)), RbBi.sub.3 S.sub.5 (Schmitz, D., et al., Naturforsch, 29b, 438-439 (1974)), Cs.sub.3 Bi.sub.7 Se.sub.12 (Cordier, G., et al., Rev. Chim. Miner. 22 676-683 (1985)), .alpha., .beta.-BaBi.sub.2 S.sub.4 (Aurivillus, B., Acta Chem. Scand. A37, 399-407 (1983)), Sr.sub.4 Bi.sub.6 Se.sub.13 (Cordier, G., et al., Rev. Chim. Miner. 22, 631-638 (1985)), BaBiSe.sub.3 (Volk, K., et al., Naturforsch. 35b, 136-140 (1980)), K.sub.3 SbSe.sub.4 (Eisenmann, B., et al., Naturforsch. 44b, 249-256 (1989)), RbSb.sub.3 Se.sub.5 (Sheldrick, W. S., et al., Z. Anorg. Allg. Chem. 557, 98-104 (1988)), Cs.sub.2 Sb.sub.4 Se.sub.8 Sheldrick, W. S., et al., Z. Anorg. Allg. Chem. 536, 114-118 (1986)), Cs.sub.3 Sb.sub.5 Se.sub.9 (Sheldrick, W. S., et al., Z. Anorg. Allg. Chem., 561, 149-156 (1988)), Ca.sub.2 Sb.sub.2 S.sub.5 (Cordier, G., et al., Rev. Chim. Miner. 18, 218-223 (1981)), Ba.sub.8 Sb.sub.6 S.sub.17 (Dorrscheidt, W., et al., Z. Naturforsch. 36B, 410-414 (1981)), and Sr.sub.3 Sb.sub.4 S.sub.9 (Cordier, G., et al., Rev. Chim. Miner. 19, 179-186 (1982)), which were prepared at high temperature by direct combination of the elements or alkali carbonates with Bi/Sb and S/Se.
The synthesis, structure and function of .beta.,.gamma.-CsBiS.sub.2 (McCarthy, T. J., et al., Chem. Mater. 5, 331-340 (1993)), KBi.sub.3 S.sub.5 (McCarthy, T. J., et al., J. Am. Chem. Soc. 117, 1294-1301 (1995)), KBi.sub.6.33 S.sub.10 (Kanatzidis, M. G., et al., Chem. Mater. 8, 1465-1474 (1996); Kanatzidis, M. G., et al., Mater. Res. Soc. Symp. Proc. 410, 37-43 (1996); Chung, D.-Y, et al., Mat. Res. Soc. Symp. Proc. 478, 333-344 (1997)), K.sub.2 Bi.sub.8 S.sub.13 (Kanatzidis, M.G., et al., Chem. Mater. 8 1465-1474 (1996); Kanatzidis, M.G., et al., Mater. Res. Soc. Symp. Proc. 410 37-43 (1996); Chung, D.-Y., et al., Mat. Res. Soc. Symp. Proc. 478 333-344 (1997)), .alpha.-K.sub.2 Bi.sub.8 Se.sub.13 (McCarthy, T. J., et al., Chem. Mater. 5, 331-340 (1993)), BaBiTe.sub.3 (Chung, D-Y., et al., J. Am. Chem. Soc., in press;
Chung, D.-Y., et al., Mat. Res. Soc. Symp. Proc. 453, 15-22 (1997)), Cs.sub.2 Sb.sub.4 S.sub.8 (McCarthy, T. J., et al., Inorg. Chem. 33, 1205-1211 (1994)), CsSbS.sub.6 (McCarthy, T. J., et al., Inorg. Chem. 33, 1205-1211 (1994)), KThSb.sub.2 Se.sub.6 (Choi, K.-S., et al., Inorg. Chem. 36, 3804-3805 (1997)) and BaLaBi.sub.2 Q.sub.6 (Q=S, Se) (Choi, K.-S., et al., Inorg. Chem. 36, 3804-3805 (1997)) were recently reported. Some of these compounds have highly promising thermoelectric properties (Kanatzidis, M. G., et al., Chem. Mater. 8, 1465-1474 (1996); Kanatzidis, M. G., et al., Mater. Res. Soc. Symp. Proc. 410, 37-43 (1996); Chung, D.-Y., et al., Mat. Res. Soc. Symp. Proc. 478, 333-344 (1997); Chung, D.-Y., et al., Mat. Res. Soc. Symp. Proc. 453, 15-22 (1997)). In a previous publication it was reported that K.sub.2 Bi.sub.8 S.sub.13 possesses significantly higher electrical conductivity (.about.10.sup.2 S/cm) at room temperature than that of its parent compound, Bi.sub.2 S.sub.3 (Gildart, L., et al., J. Phys. Chem. Solids 28, 246 (1961; Nayak, B. B., et al., J. Mater. Sci., 21, 46 (1986)) and shows unusually high thermopower (Kanatzidis, M. G., et al., Chem. Mater. 8, 1465-1474 (1996); Kanatzidis, M. G., et al., Mater. Res. Soc. Symp. Proc. 410, 37-43 (1996); Chung, D.-Y., et al., Mat. Res. Soc. Symp. Proc. 478, 333-344 (1997). The Seebeck coefficient of K.sub.2 Bi.sub.8 S.sub.13, however, differs greatly in samples of different preparation, which is believed to be due to the fact that in this compound there is occupancy disorder in some crystallographic sites between K.sup.+ and Bi.sup.3+. If the degree of disorder varies from sample to sample, it makes preparation of strictly identical samples difficult.
There is an extensive patent art. Of general interest in the preparation of such compounds are U.S. Pat. Nos. 3,352,640 to Silverman; U.S. Pat. No. 3,933,990 to Gentile et al; U.S. Pat. No. 3,940,472 to Donohue; U.S. Pat. No. 3,372,997 to Bither et al; U.S. Pat. No. 3,448,053 to Stamford et al; U.S. Pat. No. 4,576,634 to Badesha et al and U.S. Pat. No. 5,531,936 to Kanatzidis et al.